Induced responses to herbivory are ubiquitous in plants. However, whether plants can perceive and respond differentially to specialist and generalist herbivores remains unclear. Herbivory is often reduced in the introduced range of an invasive plant. Therefore, we predicted that plants collected from invasive populations might have lower or altered induced resistance responses relative to native populations. To address this hypothesis, we induced Alternanthera philoxeroides populations from native and introduced ranges with a specialist (Agasicles hygrophila), a generalist (Spodoptera litura), clipping, or jasmonate (JA) in a large common garden glasshouse experiment, and then compared their effects on plant fitness and functional traits and by challenging plants with insects of both species in bioassays.
Results/Conclusions
The negative effect of the induction treatments on the growth of the generalist herbivore was dramatically stronger, than for the specialist herbivore, which was generally unaffected. Compared to controls, aboveground biomass increased following clipping. Branching intensity and specific leaf area increased following generalist damage, whereas trichome density increased following specialist damage. Prior damage by both the generalist and specialist induced greater resistance than clipping and JA to subsequent growth of both herbivores in the bioassays. The effect of the induction treatments did not depend on the geographic origin of the plants for most plant traits. Overall, differences in the specificity of induction and effects of induced response were found. However, this variation was not generally associated with diet specialization in herbivores. Additionally, there was no evolutionary change of specificity between native and invasive populations.